The Snake River Plain of Idaho is relatively flat with the only high points in the plain
being volcanic in origin, with for example the cinder and spatter cones produced by volcanic
eruptions during the last 15,000 years in the Craters of the Moon National Monument.
These vents formed in not very explosive eruptions which ranged from completely effusive like
seen at the Fagradalsfjall volcano in 2022 to moderately explosive like seen at the Cumbre
Vieja volcano in 2021.
Craters of the Moon is one of 4 active volcanic systems which recently covered fairly large
sections of the state with a layer of black basaltic lava rock.
Yet, these effusive to moderately explosive eruptions are not the only volcanoes in the
state which could erupt again.
I am not referring to a section of the Yellowstone supervolcano that exists in Idaho, nor am
I referring to the Menan Buttes tuff cones which explosively erupted around 13,000 years
ago.
Rather, I am referring to a series of impressive and steep domes which tower over sections
of the Snake River Plain such as the Three Buttes but also a series of similar features
near the southeast corner of the state known as the Blackfoot volcanic field.
In both instances these features contain large rhyolite lava domes, representing the highest
silica content lava which is often associated with highly explosive volcanic eruptions.
The eastern Snake River Plain represents a weak area of crust where the Yellowstone Hotspot
progressed through over millions of years, producing voluminous explosive eruptions.
This hotspot like many others does not only produce volcanism where it is centered but
parts of the plume trail albeit in a weaker manner when compared to the plume center in
the direction of tectonic plate movement.
This is one possible reason why both the Three Buttes and rhyolite domes in the Blackfoot
Volcanic Field approximately align with the current location of the Yellowstone Hotspot.
In these instances, basalt magma intruded into the crust, but remained for some time,
leaching material from the surrounding crustal rock, until finally evolving into rhyolite
which at some point erupted.
In the case of the Blackfoot volcanic field, it happens to be located at a very edge of
a large region of slow tectonic spreading known as the Basin and Range province, with
large fault scarps from this process being quite prominent.
Thus, this spreading allowed for the region to act as a path of least resistance for intruding
magma, allowing for the Blackfoot volcanic field to form.
The Blackfoot volcanic field consists of three rhyolite lava domes, the largest of which
is 6700 feet long and 880 feet in height.
It also consists of two deep volcanic explosion craters termed maars, with the Gronewell Maar
measuring 2900 feet wide and 180 feet deep.
Both maars formed in the beginning of a single eruptive sequence which began 58,000 years
ago, as intruding rhyolitic magma interacted with overlying water, the water flashed to
steam and exploded in what could best be described as a several kiloton TNT equivalent blast.
As gas was released and an eruption plume towered 20 kilometers into the atmosphere,
pyroclastic flows collapsed back to the ground and scorched the landscape up to a distance
of 4 miles.
As intermittent smaller explosions continued, rhyolite lava began effusing onto the surface
along three points in a fissure at a rate of around 100 cubic meters per second.
Throughout the remainder of the 5 month long eruption, parts of the lava dome would intermittently
collapse, releasing pyroclastic flows in Merapi type vulcanian eruptions.
While the Blackfoot volcanic field is dormant and has a low rate of eruptions, it could
erupt again at some point in the most likely distant future.
The second group of rhyolite lava domes are the Three Buttes with each tower around approximately
1,000 feet above the surrounding landscape, being known as Big Southern Butte, Middle
Butte, and East Butte.
While the East Butte and Big Southern Butte are easy to explain as they were formed due
to the eruption of voluminous rhyolite lava domes in fairly explosive eruptions, with
the East Butte forming 600,000 years ago and the Big Southern Butte forming 300,000 years
ago, the Middle Butte is the odd one out.
Instead, it was formed by large scale uplift of a rhyolite composition cryptodome that
never erupted onto the surface.
While this volcanic group has not erupted in 300,000 years, I cannot truly call it extinct
quite yet so it best fits as dormant as it has only erupted three times in the last 1
million years.
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